devtool: add: drop superfluous validation for recipe name

devtool: add: drop superfluous validation for recipe name

Now that recipeutils.validate_pn() properly validates characters used in
the name, we can drop this bit checking for '/' since that's not
permitted by validate_pn(). (The FIXME comment here - that I myself
apparently wrote - is questionable since that function was clearly never
intended to allow '/', perhaps I was misled because it was broken and
did so).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>